The Welsh Government said it would review the machines' impact on farms.

In one incident in the Tregaron area of Ceredigion, an anaerobic digester leaked into the River Teifi killing 1,000 fish.

Afonydd Cymru chief executive, Dr Stephen Marsh-Smith, said more scrutiny of the industry was needed.

"At the moment anaerobic digesters aren't regulated properly, this needs to be regulated like an industrial process," he said.

"They're often set up by contractors who don't have a lot of experience of managing them and when they're up and running there should be a full-time member of staff monitoring them.

Image caption
Farmer Chris Norman employs a full time member of staff to run his digester

"Where we've found anaerobic digesters have been successful there's been bunding in place, that's a secondary containment area, to make sure nothing escapes and a full-time staff member who can manage the process day and night.

"In Wales you could set up and run one of these with half a day's training, switch it on, then you've got huge pressure, and a load of toxic stuff, very often by a water course, and we're seeing that fail."

He added: "If this happened in an industrial process, it would be stopped, the directors would be found guilty of criminal pollution and prosecuted and there would be huge fines."

What is anaerobic digestion?

It is a process by which micro-organisms break down biodegradable material in the absence of oxygen

Organic material such as manure, crops, grass or slurry is put into large containers

Once this material breaks down it produces biogas such as methane

The methane can be converted and fed into the National Grid

Chris Norman, a farmer in Pembridge, Herefordshire, has been running an anaerobic digester from his farm for the past three years.

He produces enough power to provide electricity to 850 homes, employs a full-time member of staff to run the digester and has spent an extra £40,000 on a secondary containment area, to make sure any material would be contained if anything did fail.

"I've built ramps, walls and concreted the whole area. So it won't run and escape from here. Also when building the anaerobic digester we made sure it wasn't near any water courses," he said.

"The nearest river is about half a mile from here. I don't believe I'm being particularly cautious or overly conscientious.

"I understand plants have been built on hillsides and next to rivers and this shouldn't happen. It doesn't matter how tight a farmer can be, we should all take preventative measures from digestate - the material remaining after the anaerobic digestion - getting into our rivers."

Mr Norman added: "At the end of the day the buck stops with me, so employing a full-time member of staff to run it and building this secondary containment area is worth every penny if it prevents a catastrophe happening."

The Welsh Government said the regulation and monitoring of anaerobic digesters was the responsibility of environment body Natural Resources Wales and it would take action where there is evidence of a breach.

A spokesman added: "Pollution incidents from farms have a significant detrimental impact on water quality and farmers must recognise the important responsibility they have in addressing this problem.

"We will be working with NRW to review the impact of anaerobic digesters used on farms."